5.5.2 Vertical grouping of grobs

The VerticalAlignment and VerticalAxisGroup grobs work together.
VerticalAxisGroup groups together different grobs like Staff, Lyrics,
etc. VerticalAlignment then vertically aligns the different grobs
grouped together by VerticalAxisGroup. There is usually only one
VerticalAlignment per score but every Staff, Lyrics, etc. has its own
VerticalAxisGroup.